Power MOSFETs (metal oxide semiconductor field effect transistors) are widely used in DC-DC converters. The MOSFETs are optimized to minimize losses occurring in each switching cycle. In each cycle, multiple switching phases to be passed by the MOSFET, which are different in each switching phase result in losses that can be enlarged or reduced by specific transistor parameters. During each one of these phases the entire current flows backward through the MOSFET for a short time which has a normally closed channel, with the body diode of the MOSFET forward biased. The power loss is calculated at this time by the product of current times the forward voltage of the body diode. Under these operating conditions typical body diodes have forward voltages of about 0.9V, resulting in MOSFETs with diode losses that noticeably reduce the overall efficiency of the DC-DC converter.
GaN-based HEMTs (high electron mobility transistors) are being used more widely in DC-DC converters in the future. A HEMT, also known as heterostructure FET (HFET) or modulation-doped FET (MODFET), is a field effect transistor incorporating a junction between two materials with different band gaps (i.e., a heterojunction) as the channel instead of a doped region, as is generally the case for MOSFET. HEMTs have no body diode in the conventional sense. Nevertheless, the channel opens under the gate electrode of an HEMT in the off state in the reverse direction when the potential at the drain with respect to source a small negative voltage is applied. The flux of such a voltage MOS-gated reverse diode (MGD) correlates with the difference in the threshold voltage of the HEMT. However, the threshold voltage of the HEMT cannot be made too small because otherwise the HEMT would switch on again dynamically, resulting in massive losses. This requires the forward voltage of the quasi reverse diode not be set as low as it otherwise would be beneficial for optimum efficiency.
One way the body diode forward voltage for Si-based MOSFETs (also in general) can be reduced is to connect a Schottky diode in parallel with the body diode of the transistor. This additional diode has, depending on the metal contact, a lower forward voltage of about 0.4V. As such, the power loss during phase shifts of a DC-DC converter is only half as large compared to just the body diode. Even with a low threshold voltage, contacts needed for the Schottky diode cause high reverse leakage currents at higher temperatures which can be disruptive for the application. Schottky diodes can also be used with lateral HEMTs based on III-nitride. Such conventional structures however result in a forward voltage which is limited by the material to about 1V.
A trench structure or columnar pn structures provided to the right and left of the Schottky contact can reduce the fields near the Schottky contact, resulting in reduced leakage currents. Both the trench and columnar pn concepts, however, prove difficult to implement for a Schottky contact integrated with the process of a trench MOSFET. These concepts also degrade the quality of the Schottky contact in the last process steps that are necessary for the MOSFET. The robustness of the Schottky contact with current and temperature stress in the application is also not optimal.
It would be desirable to have a structure in which the flux-voltage is further lowered and the threshold voltage is separately optimized.